Most cases of asthma are allergic in origin and are characterized by eosinophilic airway inflammation associated with mucous hypersecretion and airways hyper-responsiveness (AHR). Airway inflammation in asthma results from an allergic-type reaction to an inhaled antigen (allergen) from the environment. It is now clear that this reaction is primarily orchestrated by CD4[+] MHC class II restricted T cells that recognize and become activated by specific allergens via their T cell receptor. The asthma phenotype is thought to be primarily driven by Th2-type CD4[+] T cells; however, other CD4[+] T cell subsets, such as Th1 cells, Th17 cells and regulatory T cells (Tregs) have also been implicated in asthma pathogenesis. In this proposal, we will use MHC class II tetramers to specifically identify and study allergen-specific T cells recovered from the airways and blood following endobronchial segmental allergen challenge (SAC) in human subjects. We will compare the numbers, phenotype and function of effector and regularoty T cells recovered from two groups of allergic subjects: allergic asthmatics (AA) and allergic non-asthmatics (ANA). For unclear reasons, ANA subjects have symptomatic allergic inflammation to common aeroallergens in the skin, nose and/or eyes, but do not have symptoms of asthma. We hypothesize that there are fundamental differences in the T cell response in the lung to allergens in AA when compared to ANA subjects that account for the difference in the clinical response. In this project we will correlate the phenotypic and functioanl differences in T cell subtypes in these subjects to changes in airway physiology following SAC utilizing novel and innovative PET-CT imaging techniques. The specific aims are: 1) To determine the correlation between airway inflammation and AHR following SAC; 2) To determine the phenotype of allergen-specific and bulk CD4[+] T cells in the blood and airway following SAC; 3) To identify the mechanisms that control T cell activation and activity in the airways following SAC; 4) To determine the numbers, phenotype and function of Tregs in the airway following SAC. This proposal will define the phenotype of T cells in asthma, the mechanisms that control their activity, and the links between allergic airway inflammation and AHR.